As disclosed in U.S. Pat. No. 4,401,425 to Gable et al., a manually resettable thermal switch is used to sense the temperature of flue gases within the discharge box of a forced air gas fired furnace. The thermal switch is arranged to shut down the furnace power supply and to turn off the gas valve when it senses an over-pressure condition, caused by an over-temperature condition, in the discharge box. Once the thermal switch opens, a serviceman must be called to determine the cause of the shut down and reset the switch.
Oftentimes, the thermal switch can be tripped by events other than a vent blockage, such as high wind conditions or momentary downdrafts. In the event the building being heated remains unoccupied for a long period of time during cold weather, and a trip occurs during this time, water fixtures and pipes can freeze up and burst, causing a good deal of costly damage to the structure. Service people sometimes are not readily available, and extended delays in the serviceman's arrival during cold weather can also result in broken water pipes and fixtures. Reoccurring nuisance trips where a serviceman must be called to reset the thermal switch can also be extremely annoying as well as costly.